The present invention relates to parking disk transducer heads, in particular when there is a power loss to a disk drive.
Disk drives typically have a number of disks with a number of transducer heads supported by an actuator structure. The actuator structure is controlled by a VCM (voice coil motor). The transducer heads float or fly on a cushion of air over the disk surface, with the air flow generated by the spinning disk. When the disk stops spinning, the transducer head will land on the disk surface, potentially damaging the transducer head and/or the data on the disk. Accordingly, before the disk is stopped spinning, the transducer head is moved away from the data area of the disk, typically to a landing or parking area, where a permanent magnet can hold the transducer head in place without requiring power. The parking area may be a ramp, which requires power to exert some force to move the transducer head up the ramp to a stop next to the magnet.
In the event of a power failure in a hard disk drive, the transducer head must be unloaded from the media and parked inside the ramp stop. This function is normally known as “emergency retract” and can be performed using a blind retract or a velocity controlled retract. Since the power supply is no longer providing power, the VCM either receives power from a reservoir capacitor or the back emf (electromotive force) from the spinning spindle motor for the disk. The back emf is the voltage generated by virtue of the motor spinning. With the power lost, the motor continues to spin until it stops, generating electricity in the form of the back emf which is used to perform certain power down operations.
For a blind retract, a pre-determined retract sequence is stored in a register before loading the transducer head onto the media. When the power fails, the motor controller will use this sequence to perform retract operations. Typically, a fixed value is used. Since the transducer head may be anywhere on the disk, the value must be enough to move the transducer head all the way across the disk to the parking ramp. When the transducer head is near the parking ramp, this usually ends up taking more power than needed, with the actuator being driven for a period of time after the transducer head is already parked. Another disadvantage of this method is that the velocity of the retract operation is not well controlled which will lead to a “head bounce back” reliability issue. That is, the actuator will literally bounce off the ramp stop, then will be powered into the ramp stop again, with a smaller bounce back, giving a bouncy or vibrating landing. In the worst case scenario, the actuator is not stopped at the ramp stop, which could lead to the transducer head dropping onto the media when the hard disk drive is moved. For a blind retract using a PWM (pulse width modulation) method to save power, the PWM frequency typically used is in the frequency spectrum that is audible to the human ear. Since the parking time is longer than necessary for many transducer head locations, this causes acoustic noise longer than is necessary.
For a velocity controlled retract, detection of the transducer head in the ramp stop can be determined by sensing the current for an extended period of time. In a closed velocity control loop, the VCM current is used to regulate the velocity. When the head hits the ramp stop, the head cannot move and maximum current is forced to the motor as it tries to move the head. By detecting when the current reaches a maximum level, and remains there for a period of time, the head reaching the ramp stop can be detected. To insure the head has truly landed after multiple possible bounce backs, a pre-determined time is needed that has the same acoustic noise problem as in the blind retract.
FIG. 1 illustrates Bout and Aout, the output voltage drive signals in two directions of the VCM motor. This shows an operation with velocity control with a fixed retraction, as described in a copending application of the same assignee and one of the inventors, application Ser. No. 60/740,103, filed Nov. 28, 2005, entitled “voice coil motor control system and method using pulse width modulation.” Rather than showing the current monitor, FIG. 1 illustrates the characteristics of the back emf (bemf) for this prior method. In particular, at a point 12, power is lost and the power down sequence begins with power being applied to the VCM as illustrated by the Aout and Bout signals. The velocity of the actuator grows until it reaches a steady state value, and then sharply drops at a point 14. Point 14 reflects that the transducer head has hit the stop at the parking area, and thus is no longer moving and no longer generating the bemf. However, since the prior method continues to provide current to the VCM for a fixed retract period, the Aout and Bout signals continue to provide pulses for the period of time almost equal to the entire time period required to retract the head. As can be seen, line 14 crosses a zero speed threshold to indicate the point where, the head has stopped.